When used herein, "wet on wet" refers to a painting technique in which the drying times of the coating compositions and/or the paints applied to a canvas or other surface being painted or on which an artist is working to create a painting are delayed so as to extend the time during which the surface can be satisfactorily and creatively worked to obtain the result desired by the painter or artist.
Heretofore this technique has only been used successfully by artists using oil-based coating compositions and paints. This, however, has limitations because oil-based coating compositions and paints, particularly when used indoors, have strong and sometimes unpleasant odors and additionally are hazardous because of the solvents used in such oil-based coating compositions and paints.
Aqueous or water-based coating compositions or paints for use in such "wet on wet" painting techniques present difficulties because of their rapid drying characteristics coupled with the normal formation of a film or skin on the composition after it is coated on a surface, particularly where the coating has a thickness such that the film dries while the undercoating remains liquid or wet. Those skilled in the art will recognize that the drying of such aqueous or water-based paints occurs because of a phase reversal from liquid to solid of the applied coating material upon loss of water in the composition generally by evaporation and the agglomeration of the polymer particles of the water-based composition into a coherent film. In the case of a thicker coating of the water-based composition, this occurs first at the surface. This phase change from a continuous water phase to a continuous solid phase is a physical reaction which is generally irreversible. Therefore, it impedes the ability to further work the surface to which such paint compositions have been applied.
Slow-drying water-based coating compositions are known, such as is shown and disclosed in U.S. Pat. No. 3,374,196. In this patent a wide variety of water-based natural rubber latex coating compositions are identified and the difficulties due to rapid drying and skin formation due to the thickness of the latex compositions when applied to a surface are overcome by additives to the latex compositions that retard the rate of drying and of skin formation. The particular additive identified is a combination obtained from a physical mixture of urea and a non-polymeric water soluble glycol, such as glycerin, ethylene glycol or sorbitol. When this additive is combined with the latex compositions in the manner disclosed, a paste-like latex composition is formed which has the same rheological properties as conventional artists' oil paints. A preferred additive mixture consists of one part by weight of urea and one part by weight of glycerin formed by heating the glycerin to 250.degree. F. and dissolving the urea therein, in the form of urea pills, until the urea is fully dissolved. This specific additive delays the drying time of the paste-like natural rubber latex composition to which it is added but only about twenty (20) minutes.
Pat. '196 makes it clear that there is some synergistic effect between the urea and the glycerin for delaying drying time of the natural rubber latex composition vehicle to which they are added because this patent specifically states that such delay in drying time cannot be achieved by either of these ingredients separately. The same effect was found to apply for other normal humectants such as ethylene glycol, propylene glycol and the like.
In the present invention, contrary to U.S. Pat. No. 3,374,196, it has been found that by adjusting the ratio of glycerin, or the ratio of glycerin and other additives to a synthetic resin water-based composition such as an acrylic paint or coating material, results in the formation of a synthetic resin acrylic composition with material and substantial delays in the drying time of such synthetic resin water-based or acrylic compositions to the extent that when such improved synthetic resin acrylic compositions are coated on and worked into a surface on which a painting is being made, these substantial delays in drying time of the coating applied to the surface extend the working time for water-based paints overcoated onto the base coated surface and thus enable the painter or artist to achieve advantages or necessary painting changes and finishes on the coated surface. In this regard, not only is it possible to provide synthetic resin water-based acrylic coating compositions, but also improved paste-like synthetic resin water-based acrylic compositions, each with their respective delay in drying time for achieving these advantageous results and characteristics. However, these respective delays in drying time, as will be clear from the examples set forth hereinafter, are at least in the order of forty (40) hours or more and thus provide extended working time for overcoating the base coated surface being painted. Thus, these synthetic resin acrylic coating compositions serve well as undercoatings and are adapted to be used synergistically with either regular commercial acrylic paints or with the improved synthetic rubber paste-like acrylic compositions in accordance with the present invention to meet the requirements for use in "wet on wet" painting techniques by artists, particularly those who wish to paint indoors as well as elsewhere.
Acrylic, when used herein, is a designation for a series of colorless olefin pungent acids obtained from the oxidation of acrolein having the general formula C.sub.n H.sub.2N O.sub.2.
Acrylic paints or coating compositions are any of a group of thermoplastic synthetic resins formed by polymerizing esters of acrylic acid or methylacrylic acid. These materials are different and distinguishable from alkyd resins and natural rubber latex materials.
Any acrylic materials as above identified may be used in the formation of the improved acrylic compositions in accordance with the present invention. Such acrylic materials and other ingredients of these improved formulations are purchasable in the commercial marketplace or are readily available. These ingredients for such improved formulations may consist of one or more of the following:
Pebeo.TM. Acrylic Clear Paint PA1 Pebeo.TM. Acrylic White Paint, Extra Fine Color 118 Series 1, Blanc de Titane PA1 A humectant such as Glycerin USP 96% which has an affinity for water with stabilizing action on the water content of a material and keeps the moisture content caused by humidity within a narrow range PA1 Acrylic Flow Improver--a concentrated solution of wetting agent PA1 Pebeo.TM. Acrylic Gel--a thickening agent
These may be used and mixed in the improved formulations of the acrylic compositions in accordance with the present invention in ratios either by volume or by weight.
Thus, the general steps for the formulation of the aqueous coating compositions or paint formulations, to provide a better understanding of the invention, will now be illustrated by reference to the following examples.